Nonaqueous electrolyte secondary batteries, particularly lithium secondary batteries, lave high energy density and are thus in wide use as batteries for personal computers, mobile telephones, portable information terminals, and the like.
Nonaqueous electrolyte secondary batteries, typically lithium secondary batteries, have high energy density. Nonaqueous electrolyte secondary batteries let a large current flow and thus generate intense heat in the event of an accident such as a breakage of the battery or a breakage of the device using the battery has caused an internal short circuit or external short circuit. Nonaqueous electrolyte secondary batteries are thus required to prevent more than a certain level of heat generation to ensure a high level of safety.
Such a high level of safety is ensured typically by a method of imparting to the nonaqueous electrolyte secondary battery a shutdown function, which is a function of a separator blocking passage of ions between the anode and the cathode in the event of abnormal heat generation to prevent further heat generation. The shutdown function may be imparted to a separator by, for example, a method of using, as the separator, a porous film made of a material that is meltable in the event of abnormal heat generation. A battery including such a separator allows the porous film to be melted and made non-porous in the event of abnormal heat generation to block the ion passage for prevention of further heat generation.
An example separator having the shutdown function is a porous film containing polyolefin. Such a porous film (separator) is melted and made non-porous at a temperature of approximately 80° C. to 180° C. in the event of abnormal heat generation in the battery to block (shutdown) the ion passage for prevention of further heat generation. There have been proposed various methods for producing a porous polyolefin film having the shutdown function (see Patent Literatures 1 to 3).
To prevent a short circuit that may occur in a case where, for example, the battery generates intense heat, there have been proposed some nonaqueous electrolyte secondary battery separators that are excellent in shape stability at high temperatures. An example is a nonaqueous electrolyte secondary battery laminated separator including a laminated porous film, which is a laminate of (i) a heat-resistant porous layer and (ii) a porous film (hereinafter also referred to as “base material porous film”) containing polyolefin as a main component (base material) (see, for example, Patent Literatures 4 and 5).
As lithium secondary batteries have an increasingly wider range of applications, there have been developed, for example, (i) a lithium secondary battery having a higher energy density and (ii) a lithium secondary battery having a better output characteristic. This has led to a demand for a safer separator. Further, there is also a demand for a smaller, thinner lithium secondary battery. Therefore, separators are under development that are thinner with performance capabilities such as the output characteristic and safety maintained or improved.